An infrared absorbent composition which selectively absorbs from far infrared light to near infrared light having wavelengths of 600 nm or longer has hitherto been strongly desired for various uses but suitable compositions have not yet been obtained. The main uses of conventional infrared absorbent compositions are explained hereinbelow with regard to the following five examples.
(1) Safelight filter for infrared-sensitive light-sensitive material:
Recently, many silver halide light-sensitive materials having sensitivity from far infrared light to near infrared light having wavelengths of 700 nm or longer have been developed. For instance, an infrared sensitivity may be imparted to silver halide light-sensitive materials including ordinary photographic materials such as black-and-white photographic materials and color photographic materials and to instant type photographic materials and heat developable photographic materials in order to provide pseude color photographic materials. These materials may be used to survey resources and are capable of being exposed using a diode emitting infrared light.
A panchromatic safelight filter has previously been used for such infrared-sensitive light-sensitive materials.
(2) Controlling the growth of plants:
The formation, growth and differentiation of plants, such as the germination of seeds, the growth of stalks, the expansion of leaves, the formation of the tubers of flower buds, etc., are known to be influenced by light. This influence of light on plant formation has been previously studied.
A plastic film capable of selectively absorbing light having wavelengths of 700 nm or longer has the effect of, for example, delaying the time of earing and controlling the growth of the ear when it is used to cover the crops at a specific time (see, Katsumi Inada, Shokubutsu no Kaqaku Chosetsu (Chemical Control of Plants), Vol, 6, No.1, (1971).
(3) Interception of thermal radiation:
The radiation energy of the sun contains light in the range of from near infrared to far infrared having wavelengths of 800 nm or longer. When this light is absorbed by a substance it is converted into heat energy. The greater part of the energy distribution of such light which is converted into heat energy is concentrated in the near infrared region having wavelengths of from 800 to 2,000 nm. Accordingly, a film capable of selectively absorbing near infrared rays is very effective for intercepting the solar heat. The use of such a film in a room would allow the temperature increase in the room to be sufficiently controlled while allowing the entrance of a sufficient amount of visible light. These films can be applied to greenhouses for gardening as well as to houses, offices, stores, and to the windows of cars or planes, etc.
A plastic film having a vapor-deposited thin metal layer and glass having dispersed therein an inorganic compound such as FeO, etc., have previously been used for intercepting the thermal radiation of the sun.
(4) Cut filter for infrared rays harmful to human eyes:
One of the main uses of an infrared cut filter is in glasses for protecting human eyes from light such as the above-mentioned harmful infrared rays. For example, the infrared cut filter may be used in sun glasses, protective glasses for welders, etc.
(5) Infrared cut filter for a semiconductor light-receiving element:
A silicon photo diode (hereinafter referred to as SPD) is currently used as a light-receiving element for a photodetector which may be used in an automatic exposure meter of a camera. For example, JP-A-61-11704 shows graphs of the relative luminosity curves and the relative values (spectral sensitivity) of the output of an SPD for each wavelength. (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
When using an SPD for an exposure meter, it is necessary to cut the light of the infrared region, which the human eye is insensitive to, such that the spectral sensitivity curve of the SPD shown in FIG. 4 of the aforesaid patent application becomes similar to the relative sensitivity curve shown therein. In particular, since light of the wavelength region of from 700 to 1,100 nm causes a large output of the SPD and the human eye is insensitive to light of this region, the light of this region is a factor for causing an error in the exposure meter. Thus, if an infrared absorbent plastic film which absorbs less visible light and absorbs infrared light over the entire region of from 700 to 1,100 nm can be used, it is clear that light of the visible wavelength region will be largely transmitted and the output of the SPD by visible light is increased, whereby the performance of exposure meter is greatly improved.
Hitherto, an SPD having an infrared cut filter composed of glass containing an inorganic infrared absorbent at the front surface thereof has been used as a kind of photodetector.
However, almost all of the conventional organic dye series infrared absorbents are inferior in light fastness and heat resistance and are insufficient for practical use.
Also, conventional filters which are used for the aforesaid various uses have the following disadvantages.
First, since conventional panchromatic safelight filter for the aforesaid use (1) not only partially transmits green light having high luminosity but also transmits a large amount of infrared light, the use of such filter results in light fog. Accordingly, these materials cannot be sufficiently used as a safelight for infrared light-sensitive materials.
Also, since a plastic film having a vapor-deposited metal layer or a glass having dispersed therein FeO for the aforesaid use (3) strongly absorbs not only infrared light but also visible light, the illuminance in the inside of the room, etc., shaded by such a film or a glass plate is lowered. Thus, the use of such a plastic film for agriculture is inadequate since it causes an absolute shortage of sunshine.
Furthermore, an infrared ray cut filter composed of glass using an inorganic infrared absorbent used for the aforesaid use (5) is comparatively fast to heat and light but shows a low light transmittance in the visible wavelength region. Therefore, the reduction in light transmittance is compensated by increasing the sensitivity of the SPD. However, increasing the sensitivity of the SPD results in an increase of the leak current which results in errors in the photodetector, and also results in a large problem from the point of reliability. Also, since the infrared ray cut filter is composed of an inorganic substance, the filter is lacking in flexibility from the viewpoint of the production of the photodetector and it is difficult at present to improve the production process theeof. Still further, the infrared cut filter composed of an inorganic substance is high in production cost, which results in greatly increasing the cost of the photodetector.
As described above, a photodetector using the inorganic cut filter may have a spectral sensitivity near the relative luminosity curve thereof, but it has serious disadvantages from the viewpoints of reducing the operation performance of the photodetector, increasing the production cost, and improving the production process thereof.
Also, a near infrared absorbent plastic film using a complex as the infrared absorbent has deficient solubility in an organic solvent, which is a large problem in the case of forming a thin plastic film thereof.
Moreover, in the uses described above, it is desired in an SPD filter that the filter is composed of a very thin film and shows a good infrared absorption efficiency. For this purpose, it is required that a large amount of an infrared absorbent is dispersed in a resin for the film but an infrared absorbent having less solubility in an organic solvent cannot meet the purpose.
Furthermore, a conventional near infrared absorbent plastic film using a metal complex as the infrared absorbent has a short maximum absorption wavelength and, in particular, is unsuitable for use in the light-receiving element of a semiconductor laser.